I believe its Glenn, that is who Negan killed in the comics, and his death is to important for future story lines. Based off where the RV is behind Negan it is the the left of Rick, giving us the line up of Glenn, Abraham, Rosita, Daryl and Michonne, I doubt it is Daryl, people talk about Norman Reedus' new show as evidence its him, his show is a 6 episode series of him and a friend riding motorcycles, not a scripted show, he can knock that out in a couple weeks in the shows down time, so that is not strong enough evidence for Daryl.